In recent years, from the viewpoint of improving fuel efficiency and collision safety of automobiles, automobile body materials have been required to achieve higher strength so that automobile bodies can be made thinner, more lightweight, and stronger. Accordingly, application of high-strength steel sheets to automobiles has been encouraged.
In general, steel sheets for automobiles are painted and used. Prior to painting, a chemical conversion treatment known as phosphating is performed. This chemical conversion treatment is one of processes critical for ensuring corrosion resistance after painting.
Addition of Si and Mn is effective for increasing the strength and ductility of steel sheets. However, Si and Mn undergo oxidation during continuous annealing even when annealing is conducted in a reducing (Fe-oxide reducing) N2+H2 gas atmosphere that does not induce oxidation of Fe. Due to this oxidation, surface oxides selectively containing Si and Mn (SiO2, MnO, etc., hereinafter referred to as selective surface oxides) occur in the outermost surface layers of steel sheets. Selective surface oxides inhibit reaction of forming chemical conversion coatings during the chemical conversion treatment. Accordingly, due to the presence of selective surface oxides, small regions (also referred to as uncoated regions) where no chemical conversion coatings are formed occur. As such, presence of selective surface oxides decreases the chemical convertibility of steel sheets. Patent Literatures 1 to 6 are available as the conventional techniques related to high-strength steel sheets.